The yeast Saccharomyces cerevisiae is an attractive system for the analysis of genes conserved during eukaryotic evolution. We have used this system t determine the function of cyclophilins, a family of highly conserved, ubiquitously expressed proteins which bind the immunosuppressor cyclosporin A with high affinity. We isolated two yeast genes, CPR1 and CPR3, encoding cyclophilins by cross hybridization with a rat cyclophilin gene. Expressio of both genes in E. coli demonstrates that they encode CsA binding proteins We have studied disruption mutants of both genes as well as of another cyclophilin gene, CPR2, and of the FK506 binding protein gene, FPR1. The only phenotype observed was the inability of cpr3 mutants to grow on lactat at 37 degrees C. Recent work suggests that the defect lies in the folding or refolding of this enzyme. We have isolated mutants with dominant suppressors of this phenotype, and have prepared a genomic library from one of these strains to clone the suppressor gene by complementation. To extend such analysis to other conserved genes, we investigated a system for selecting conserved genes in yeast. Recombination in yeast is strongly dependent on homology. Accordingly, we expect that transformation of yeast by a plasmid containing a mammalian gene will result in integration of the plasmid at the homologous, conserved yeast gene. To test this, a plasmid containing a rat cyclophilin gene was used to transform yeast; all transformants had integrated at the homologous CPR3 locus. While the lengt and homology dependence of this recombination must still be determined, thi system offers the prospect of systematically identifying conserved genes by transforming yeast with mammalian cDNA libraries.